It is well known that microcircuit fabrication requires precisely controlled quantities of impurities be introduced into very small regions of the silicon substrate, which are subsequently interconnected to create components and very large scale integration (VLSI) or ultra large scale integration (ULSI) circuits. Equally well known is that the patterns that define such regions are typically created by optical lithographic processes, which involve the use of a mask and radiation, such as ultraviolet light, electrons or x-rays, to expose a pattern in the photo resist material. The exposed patterns in the photo resist are formed when the wafer undergoes the subsequent development step, and protect the substrate regions that they cover. Locations from which photo resist has been removed can then be subjected to a variety of subsequent processing steps.
In today's sub-micron technologies, the degree of resolution that can be achieved by such lithographic processes is an important factor in consistently printing minimum size images. Thus, the fabrication of increasingly smaller features on VLSI relies on the availability of increasingly higher resolution lithography equipment or processes. This higher resolution may be achieved in several ways. For example, the illuminating wavelength can be decreased, or the numerical aperture of the system lens can be increased. The contrast of the photo resist can also be increased, by modifying the photo resist chemistry, by creating entirely new resists, or by using contrast enhancement layers, which allows a a smaller modulation transfer function to produce adequate images. Alternatively, the coherence of the optical system can be adjusted.
The degree of resolution has become even more critical in sub-micron circuits with features less than 0.5 .mu.m. As features sizes have become smaller, difficulty in controlling the appropriate amount of photo resist exposure has increased due to stray light problems associated with patterning these smaller features. In some cases, over exposure of the desired photo resist area may occur, and in other cases, under exposure of the photo resist area may occur. In either case, critical dimension (CD) line width control becomes more difficult.
Optical lithography for deep sub-micron integrated circuits with feature sizes less than 350 nm (0.35 .mu.m) requires shorter wavelength exposure (365 nm or 248 nm) of the photo resist materials used for defining circuits. The use of shorter wavelengths, where the photo resist material is transparent, results in a significant pattern resolution dependence on substrate reflectivity of the stray light. Accurate critical dimension (CD) line width control, therefore, requires nullifying the stray light from the reflective substrate.
To reduce the amount of stray light, organic and inorganic anti-reflective coatings (ARC)and anti-reflective layers (ARL) have been developed. The organic materials are typically spin coated onto the substrate, resulting in a planarization of previously defined circuit features. This planarization effect, however, results in significant thickness variations and difficulties in pattern transfer (i.e., etching). The inorganic ARC and ARL materials are typically silicon rich amorphous silicon-oxy-nitride deposited by plasma enhanced chemical vapor deposition using silane nitrous-oxide chemistry. While the resultant inorganic material has conformal step coverage for improved pattern transfer performance, it, as well as the organic material discussed above, contains amine groups in their matrix, which are not compatible with most deep ultraviolet (248 nm)photo resist materials.
Nevertheless, silicon rich silicon oxy-nitride thin films are gaining interest for use as an ARC for enhanced lithography performance in the I-line (365 nm) and the deep ultraviolet light regime (248 nm). These silicon oxy-nitride (SION) thin films involve basically two schemes, namely an interference scheme and a total absorbance scheme. The interference scheme uses phase shift cancellation by tuning the film thickness and optical properties so that the wave length of the reflected light is out of phase with the source light. The total absorbance scheme uses a layered film where the optical properties of the top layer are tuned to match those of the photo resist, the bottom layer's optical properties are tuned for high absorbance, and the middle layer(s) is a transition layer. The optical properties of SiON are varied by adjusting the deposition chemistry for the bottom, middle, and top layers, respectively. While these schemes can work quite well to reduce the amount of stray light, they do require extremely tight control of the deposition chemistry and resultant film thickness and thickness uniformity, all of which can be difficult to achieve.